High-speed optical transmission requires high standard coupling between fibers and photonic integrated circuits (PICs). However most photonic integrated circuits operate at a specific polarization state, usually polarization of the fundamental mode. This is different from the situation in single mode fiber where radial symmetry guarantees the equivalence of different polarization orientations.
Inside a PIC, higher order modes may be excited as a consequence of imperfections such as side wall roughness, and as a consequence of reflections, transitions between multimode and single mode regions, and other interactions of the optical signal with the mechanical structure of the PIC.
Generally, integrated waveguides have two orthogonal transverse polarization states, TE and TM, each of which has a lowest order mode, conventionally denoted TE0 and TM0, respectively. The TE0 and TM0 mode may also be referred as fundamental TE and TM modes, respectfully. Between TE0 and TM0 modes, the mode that has a lower mode number (i.e., the largest effective refractive index) may be referred to as the fundamental mode of the waveguide.
Higher order TE and TM modes may generally be suppressed by tapering down the waveguide to a single mode region, as they become leaky modes. However, it may be more difficult to get rid of the TM0 mode as its effective index in the waveguide may be close to that of the TE0 mode.
Several techniques to eliminate a TM0 mode have been suggested. One approach is to use a directional coupler to couple TM0 mode out of the waveguide. However this method is wavelength sensitive.
Another approach is to place metal atop of the waveguide to absorb the TM0 mode. However, this approach may not be compatible with standard CMOS-compatible processes and additionally may introduce an undesired extra loss for the TE0 mode.
There is a need for systems and methods that maintain optical signals propagating in photonic integrated circuits in well-defined polarization states and/or specific modes.